1. Field of the Invention
The present invention relates to a heating roller assembly disposed in a fusing unit of an image forming apparatus, such as printer, photocopier, multifunction apparatus. More particularly, the present invention relates to a heating roller assembly constituting a fusing unit.
2. Description of the Related Art
In general, a fusing unit of an image forming apparatus comprises a heating roller to heat a sheet of printing paper at a fusing temperature and a pressure roller rotated in contact with the heating roller to apply a pressure to the printing paper.
The heating roller is divided into a resistance heating roller and an inductive heating roller according to a heating method. The inductive heating roller is coming into increasing use in recent years in view of a high reliability and fusing efficiency.
The inductive heating roller has a metallic heating roller pipe wound by a copper coil. The inductive heating roller indirectly heats the heating roller pipe using an electromagnetic force generated by electrifying the coil. For example, if an alternating current (AC) of several tens or hundreds of Hz is applied to the coil from a power circuit, an AC magnetic flux is generated in the heating roller pipe wound by the coil such that an inductive current is generated in a circumferential direction of the heating roller pipe. The inductive current generates a Joule's heat and thus low frequency inductive heat raises the temperature of the heating roller pipe to a temperature (150˜200° C.) suitable for the fusing process.
Since a portion of the heating roller pipe where the coil is closely wound has a higher AC magnetic flux than a portion where the coil is sparsely wound, the portion where the coil is closely wound emits a large amount of heat. The thermal temperature of the emitted heat is controlled by adjusting the number of turns of the coil. The inductive heating method is disclosed in U.S. Pat. No. 6,341,211 and JP Patent Laid-open application 2000-029332.
However, if the heating roller pipe remains at a high temperature for a long time, a bearing supporting the heating roller pipe is subjected to the heat of the heating roller pipe and overheats. Therefore, the bearing may be damaged. Also, if an excessive heat is transmitted to the bearing, a bearing supporting member, which is made of a resin material, may melt. If the heat from the overheated bearing is transmitted back to the heating roller pipe, a difference between the temperature of a center potion of the heating roller pipe and the opposite ends can occur, which causes a hot offset.